1. Field of the Invention
The present invention relates generally to electrical cable systems, and, more particularly, to a flat multiconductor cable assembly which is installed on a floor substrate beneath carpeting.
2. Description of the Prior Art
One type of undercarpet cable system includes a flat multiconductor cable which is assembled between a plastic shield and a metallic shield. The cable assembly, comprising the cable and its two protective shields, is installed between a floor and overlying carpeting. The multiconductor cable includes a plurality of flat electrical conductors which are contained in a casing comprised of a thin sheet of electrical insulation. The plastic shield provides a cushion for the multiconductor cable so as to resist the abrasion and possible piercing of the cable insulation by projections extending upwardly from the floor, such projections being especially prevalent if the floor is made of concrete or a similar coarse building material. The metallic shield is provided to resist piercing of the cable insulation by an object inserted through the carpet. By electrically grounding the metallic shield, any electrically conductive object which may pierce the metallic shield and contact a "hot", i.e., electrically energized, conductor of the multiconductor cable will be grounded so as to protect a person who contacts the object from electrical hazard.
In order to maintain the shield grounded despite possible mechanical discontinuity as occasioned by cutting thereof and to avoid electrical hazards that may be caused by misalignment of the shield and the cable, or relative movement therebetween, the shield has successive extents that are respectively unsecured and secured to the cable as described in U.S. Pat. No. 4,283,593, entitled "Multiconductor Cable", issued on Aug. 11, 1981 and assigned to the same assignee as is the present invention. Thus, electrical connection of the shield to the cable ground conductor is made redundantly at each such secured extent of the shield whereby physical continuity of the shield may be interrupted without interrupting electrical continuity of the remnant shield to ground.
In order to provide cable flexibility and to maintain a thin profile of the cable assembly, as well as to permit ease of splicing and tapping which requires cutting or slicing of the metallic shield, the metallic shield is typically made of ductile material, such as copper, which may be selected to be on the order of several mils thick. Selection of such a thin metallic shield may not only provide minimal resistance to piercing from sharp objects but also minimum heat dissipating and electrical transfer capabilities. Sharp objects that may inadvertently be caused to penetrate the insulative cable casing and contact a "hot" conductor can then cause a localized heating of the shield around the object. Insufficient heat and electrical transfer can result in localized melting of the shield or complete sublimation thereof, leaving the "hot" object exposed and potentially dangerous.
In addition, selecting a thin cable assembly may result in a structure in which the metallic shield is relatively close to the cable conductors producing thereby an undesirably high capacitance and a high leakage current therebetween.